Currently used water treatment systems are generally based on either mechanical, chemical, biological, ion exchange and/or thermal treatment. Typical examples of these treatments are filtering and centrifuge treatments; flocking, use of resin beds and evaporation of water or volatile components respectively. The thermal treatment takes up a comparatively large share of the market of water treatment systems. In fact, a lot of waters exist that can only be treated effectively using thermal treatment, because of the presence of components that make chemical, ion-exchange or mechanical treatment impossible. Examples of such waters are waters that contain very small particles, biological active substances, or dissolved solids. This limitation can relate to the actual physical limitations of these treatment technologies, but can also be of an economic nature. A very high consumption of electrical energy typically prohibits an economically feasible use.
Two major types of thermal treatment systems are known, which are vapour-compression and multi-Effect Distillation (MED). In vapour compression, vapour that is formed as a result of a boiling process, is compressed in a mechanically driven compressor, and is used to pre-heat the incoming feed flow. This type of vapour-compression evaporator has a high capital cost, and is driven by electrical energy, and are economically applicable for higher concentrations of water, and for medium/high water volumes. In MED, use is made of high-grade thermal energy sources, like superheated steam. The thermal energy thereof is reused several times to evaporate water, in a sequence of ‘effects’ or stages that are sequentially operated at a lower pressure. Vapour generated in a first stage by means of evaporation from feed water is condensed against a condensation wall in a subsequent stage. The latent heat is transmitted again to feed water that is therewith evaporated. This reuse should in itself lead to higher thermal efficiency, but their typical structure, made out of metals like stainless steel, is sensitive to corrosion and requires high maintenance demands.
A variation on the MED principle is multi-effect membrane distillation, such as known from WO2005/089914A1. Herein, the condensation wall is arranged adjacent to a liquid channel, which again is closed by means of a membrane. Evaporation of the liquid allows the vapour to leave the liquid channel via the membrane. Both an implementation based on circular extension of foils and an implementation based on parallel arranged foils is presented. This variation of MED creates a bigger contact area between vapour and liquid and seems a suitable solution. However, the migration of vapour through the membranes tends to cause depositions of salts or organics-based fouling components. Furthermore, the membrane needs to be positioned and fixed by a frame. Both the frame and the membrane may be made of polymer material. Given the high temperatures up to the boiling point and the lower pressures, the polymer material is subject to expansion. This tends to cause reliability and maintenance issues.
Another variation on the MED treatment system is known from US2014/0042009A1. Herein the membranes have been replaced by layers of wicking material. More particularly, such layers allow the feed water to flow through the layer. As a consequence, no membrane is needed, which simplifies the construction. In the said application, the distance between the wicking layer with the feed water and the condensation wall is reduced to a small gap and a circular configuration is chosen. Some discrete spacer is arranged into the gap so as to avoid that the wicking layer would attach to the condensation wall. Clearly, in this configuration of a plurality of effects within a single stack or roll of foils, no gradually reducing pressure can be applied. This is a major limitation, as the reduction in pressure from stage to stage is one of the drivers to continue the distillation process. Moreover, notwithstanding the observations, it appears that there is still a risk of carry-over of feed water to the distillate, since the two liquid streams are arranged at the surfaces that face each other and are merely separated by the gap. It would of course be possible to enlarge the size of the gap between the wicking layer and the condensation wall, but this immediately leads to a reduction in the effective surface area per unit of volume.
A further system is known from US2014/0332365A1. This application discloses a distiller with an evaporation surface for evaporating liquid into vapour. Around a vertical central longitudinal axis alternating evaporation channels and condenser channels are present. Each of said evaporation channels has two opposed evaporation surfaces. Each condenser channel is provided with two opposed condensing surfaces. A motorized rotary assembly is present including a rotor and a pump for applying the heated liquid from a sump at a bottom of the housing to a moving applicator device that applies heated liquid in a thin film onto the evaporation surfaces with wipers. In this system, the evaporation channels constitute closed chambers. Between the closed chambers, the condensing channels extend. The heating liquid (i.e. the feed) and the condensing vapour running in the same direction, from top to bottom.
As is visible in FIG. 8 of the application, both the evaporating channel and the condensing channel are provided with a variety of structures and elements, referred to as vertical irrigation channels, elongate retaining structures having a corrugated shape and corrugations to provide elongate vertically extending or upright retaining grooves or recesses. This makes the construction, together with the rotor for rotation of liquid a complex and expensive. Apparently, these structures and the rotary movement are needed so as to ensure that liquid films will be retained at the evaporating and condensing surfaces during operation. Moreover, the structures seem to increase adhesion and deposit of contaminants in the feed within the evaporation channel. As a consequence, there is quickly a need for cleaning, unless the use is limited to liquids that are rather pure mixtures. As such, the effective operation time of the apparatus is deemed to be rather low.